lagenorhynque

trait Monoid[A] {
  def mempty: A
  def mappend(x: A, y: A): A
  def mconcat(xs: Seq[A]): A = xs.foldLeft(mempty)(mappend)
}

object MonoidTest {
  def sum[A: Monoid](xs: A*): A = implicitly[Monoid[A]].mconcat(xs)

  implicit def numericMonoid[A: Numeric] = new Monoid[A] {

lagenorhynque

import scala.annotation.tailrec

object Fibonacci {
  @tailrec
  def fibo(i: Int, prev: BigInt = 1, curr: BigInt = 0): BigInt = i match {
    case 0 => 0
    case i if i < 2 => prev + curr
    case _ => fibo(i - 1, curr, prev + curr)
  }

lagenorhynque

Functors, Monads, Applicatives – can be so simple | de.velopmind | The Det about Programming

補足説明を加えて簡単に整理してみた。

Functor: Scalaのmap

(A => B) => (C[A] => C[B])

Monad: ScalaのflatMap

(A => C[B]) => (C[A] => C[B])

lagenorhynque

module MonadTransformers where

import Control.Monad.Trans.Class
import Control.Monad.Trans.Maybe

data User = User {
  id' :: Int,
  firstName :: Maybe String,
  lastName :: Maybe String
} deriving Show

lagenorhynque

(ns random-alphanumeric-generator)

(def alphanumeric "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890")

(defn rand-alphanumeric [len]
  (repeatedly len #(rand-nth alphanumeric)))

(->> (repeatedly #(rand-alphanumeric 5))
     (filter (fn [[a _ _ _ b]] (= a b)))
     (map #(apply str %))

lagenorhynque

lagenorhynque

;;;; cf. http://riktor.hatenablog.com/entry/2012/07/20/224806

(defn fib [n]
  (if (or (= n 0) (= n 1))
    1
    (+ (fib (- n 1))
       (fib (- n 2)))))

(let [memo (atom {})]
  (defn fib-memoized [n]

lagenorhynque

import scala.collection.mutable.ListBuffer

val messageBoxes = Array.fill(3)(ListBuffer[String]())
messageBoxes(0) += "message1"
messageBoxes(0) += "message2"
messageBoxes(0) += "message4"
messageBoxes(0) += "message5"
messageBoxes(1) += "message1"
messageBoxes(1) += "message2"
messageBoxes(1) += "message3"

lagenorhynque

;;;; cf. http://riktor.hatenablog.com/entry/2012/07/22/223937

(def counter
  (let [cnt (atom 0)]
    (fn [] (swap! cnt inc))))

(counter)
(counter)
(counter)

lagenorhynque

;;;; cf. http://riktor.hatenablog.com/entry/2012/07/25/001118

(first '(1 2 3))
(first '((1 2 3) 4 5 6))
(rest '(1 2 3))
(rest '(1 2 (3 4 5)))

(cons 1 '(2 3))
(cons '(1 2 3) '((4 5 6)))
(cons 1 nil)